Label Distribution Protocol (LDP) enables peer label switch routers (LSRs) in a Multi-Protocol Label Switching (MPLS) network to exchange label binding information for supporting hop-by-hop forwarding. Various Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode, Ethernet and the like) may be emulated over an MPLS backbone by encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting them over a pseudowire (PW). Various protocols for establishing and maintaining pseudowires using extensions to LDP, for encapsulating Layer 2 PDUs and so on are described in more detail in various Internet Engineering Task Force (IETF) Requests for Comment (RFC), such as RFC4447 and related documents.
A Multi-Segment PW (MS-PW) is a set of two or more contiguous PW segments stitched together that behave and function as a single point-to-point PW. A MS-PW enables service providers to extend the reach of PWs across multiple Packet Switch Network (PSN) domains.
In a typical MS-PW implementation, a first service provider edge (S-PE) device acts as gateway for a first group of Terminating Provider Edge (T-PE) devices in a first autonomous system (AS). The S-PE device communicates with a second S-PE device in a second AS, which in turn is acting as a gateway for a second group of T-PE devices in the second AS. The first and second S-PEs are switching PEs connecting the first and second two autonomous systems, where two segments of a MS-PW are stitched together. Each S-PE maintains both control plane and data plane states for each MS-PW and participates in all Operations, Administration and Maintenance (OAM) for the MS-PWs.
Unfortunately, the typical implementation suffers from a number of limitations that result in poor scalability and other problems, such as:
(1) A S-PE needs to allocate labels from its global label space for setting up each MS-PW routed through that S-PE. Since the S-PE label space is shared by many MPLS based applications including the MPLS based PSN tunnels, the S-PE needs to maintain a high volume of control plane and data plane states for the MS-PWs stitched at the S-PE.
(2) Since every S-PE does PW segment stitching, every S-PE must participate in PW OAM functions.
(3) The MS-PW set-up and maintenance procedure causes control plane congestion at S-PEs. For example, an S-PE needs to participate in PW Status Signaling for various maintenance procedures on each MS-PW routed through it. Further, if a S-PE needs to handle network event such as failure of PW next-hops or requires re-routing then it incurs a very high volume of control plane exchanges between participating S-PE devices. LDP is the default signaling protocol for PWs and is a TCP based protocol. High volume of LDP protocol exchanges causes TCP congestion at S-PEs and impacts other essential services that use same LDP session.
(4) Each S-PE needs to maintain an ever larger number of control plane states as the number of PWs routed through the S-PE grows. The control plane has limited memory and thus limits the number of MS-PW services that can be offered by an AS.